Some applications of GaN-based LEDs include displays, indication lights, LCD backlights, and general illumination, and luminous efficiencies of such LEDs increase year by year. To improve the drive power and luminous efficiency of the LED, some manufacturers have designed a thin-film chip architecture based on substrate transfer technology, of which the most typical one is the vertical LED chip. The manufacturing method is to epitaxially grow the GaN-based light-emitting material on a growth substrate, such as sapphire, and remove the growth substrate after the light-emitting material layer is bonded to a permanent substrate, such as a semiconductor or a metal substrate via bonding or electroplating. After that, the nitrogen-polar n-GaN top surface is roughened using a wet etch to create micro-pyramid structures in the surface, thus greatly improving the light extraction efficiency. Furthermore, a reflector is provided between the light-emitting material layer and the permanent substrate to facilitate light extraction at the top side by reflecting the downward emissive light.
However, although forming the pyramid-shaped roughened surface on the exposed surface is achievable, the above-described vertical thin-film chip essentially has the top surface as the only light exit surface, since the light emitted from the sides of the relatively-thin epitaxial layers is almost negligible. Therefore, any reflected light must exit through the epitaxial layers and is partially absorbed, resulting in lowered light extraction efficiency.
In contrast to the vertical chip, the conventional horizontal chip, by adopting a transparent substrate structure, consists of at least one front light-emitting surface and four lateral light-emitting surfaces, of which the lateral surfaces emit the most light. Therefore, if the transparent substrate structure of the horizontal chip and the pyramid-shaped roughened surface of the vertical chip are combined together, the extraction efficiency can be improved. Nevertheless, in a conventional horizontal chip based on the transparent substrate structure, a transparent conductive layer, such as an indium tin oxide (ITO) layer, is typically provided as a p-type conductive window, the resistance of which is higher than the metal, thus limiting the current expansion under a high current operation since the lateral transfer resistance is relatively large. For this reason, a metal electrode is required in the light-emitting area, which reduces the extraction efficiency by blocking the light.
What is needed is an LED design that has increased light extraction efficiency.